Ultra-thin film superconducting tapes

ABSTRACT

An ultra-thin film superconducting tape and method for fabricating same is disclosed. Embodiments are directed to a superconducting tape being fabricated by processes which include removing a portion of the superconducting tape&#39;s substrate subsequent the substrate&#39;s initial formation, whereby a thickness of the superconducting tape is reduced to 15-80 μm.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application is a divisional of U.S. patent application Ser. No.15/573,632, filed on Nov. 13, 2017, issuing on Apr. 19, 2022 as U.S.Pat. No. 11,309,480, which is a national phase of PCT/US16/31838, filedMay 11, 2016, which claims priority to U.S. provisional patentapplication No. 62/159,690, filed on May 11, 2015, both of which arehereby incorporated herein by reference in their entirety.

GOVERNMENT SPONSORSHIP

None.

FIELD OF THE DISCLOSURE

This disclosure relates to superconducting tapes, and more specificallyto ultra-thin film type superconducting tapes.

BACKGROUND

Several materials systems are being developed to solve the loomingproblems associated with energy generation, transmission, conversion,storage, and use. Superconductors are a unique system that provides asolution across a broad spectrum of energy problems. Superconductorsenable high efficiencies in generators, power transmission cables,motors, transformers and energy storage. Furthermore, superconductorstranscend applications beyond energy to medicine, particle physics,communications, and transportation.

Superconducting tapes are becoming more and more popular. This is inpart due to successful fabrication techniques that create epitaxial,single-crystal-like thin films on polycrystalline substrates (Y. Iijima,et al., Physica C 185, 1959 (1991); X. D. Wu, et al., Appl. Phys. Lett.67, 2397 (1995); A. Goyal, et al., Appl. Phys. Lett. 69 (1996) p. 1795;V. Selvamanickam, et al., “High Performance 2G wire: From R&D toPilot-scale Manufacturing”, IEEE Trans. Appl. Supercond. 19, 3225(2009)). In this technique, a thin film of materials having a rock saltcrystal structure (e.g., MgO) is deposited by ion-beam assisteddeposition over flexible, polycrystalline substrates. Superconductingfilms that are processed by this technique exhibit critical currentdensities comparable to that achieved in epitaxial films grown on singlecrystal substrates. Using this technique, several institutions havedemonstrated pilot-scale manufacturing of superconducting compositetapes. Remarkably, single crystal-like epitaxial films are now beingmanufactured at lengths exceeding 1 km using a polycrystalline substratebase.

FIGS. 1A-1B illustrate a schematic and cross-sectional microstructure ofa prior art high-temperature superconducting (HTS) thin film tape madevia the afore-mentioned process. The tape typically includes severaloxide films positioned on a metallic substrate and capped with silverand copper overlayers. A typical 2G HTS tape is around 0.1 to 0.2 mmthick, which is quite thick. The substrate itself comprises most of thetape's thickness, i.e., between 50 and 65%. For example, today,superconductor tape manufacturers typically use substrates ranging from0.05 mm to 0.1 mm thick. The reason for this is to provide the tape withstrength and stability during the extreme manufacturing conditions. Forexample, during manufacturing the tape is processed and subjected totemperatures of about 800° C. and high tensions on the order of a fewNewtons. Additionally, because of challenges with thin substrates toachieve a low surface roughness and due to susceptibility to damage, thequality of the buffer and superconductor films may be compromised inthin tapes. It is noted that the thicknesses of the various layers inFIG. 1A are for illustrative purposes only and are not drawn to scale.

One major goal in superconducting tape manufacturing is to reduce theoverall thickness of a superconducting tape. A 50% reduction in thethickness of the tape can increase the engineering current density(J_(e))(i.e., critical current per unit cross section of the entiretape) by 50%. In one implementation, this increase in current densitycan result in higher ampere-turns in a coil, since more turns ofsuperconducting tape can be fitted into a given coil volume. The resultwould be a higher magnetic field that can be generated by the coil,which is important for most superconductor applications such assuperconducting magnetic energy storage (SMES) systems, high-fieldmagnets, nuclear magnetic resonance spectroscopy (NMR), magneticresonance imaging (MRI), accelerators, generators and motors. In anotherimplementation, thinner superconductor tapes can improve high currentcables that are wound using these tapes on a flexiblenon-superconducting core (D. C. van der Laan, Supercond. Sci. Technol.22, 065013 (2009)) or fabricated using a stack of tapes. For example,today, high current cables are made with 0.1 mm thick tapes. Due to thesheer size of the tape, the tape must be wound around a core having adiameter at least around 3 mm. However, because of the large core, theoverall current density of the cable is severely compromised from adesired ˜400 A/mm² at 4.2 K, 20 T (stand-alone tape) to levels of ˜100A/mm² at 4.2 K, 20 T. If thinner superconductor tapes can bemanufactured, a smaller diameter core could be used which would resultin a significantly higher overall current density.

Reducing superconductor tape thickness is challenging. About 97% of asuperconductor tape is comprised of the substrate and the copperstabilizer. The copper stabilizer is not a viable target for reducingthickness, because it requires a sufficient thickness to providestabilization of the conductor in the event of over current conditions.Hence, the only practical target for reducing tape thickness is thesubstrate. But fabricating a tape while using a thinner substratepresents several problems. First, tapes that are made on thin substratescan snap and break at the required high processing temperatures andtensions, which can greatly diminish the manufacturing yield. Second,every single step of the complex tape manufacturing process would needto be modified to accommodate different starting substrate thicknesses.Third, a tape manufacturer would need to produce several types of tapes,each with a different starting substrate thickness, to meet therequirements of different applications. This would increase themanufacturer's costs of production.

Thus, there is need in the art for methods and systems that can achieveultra-thin film superconducting tapes.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary, as well as the following detailed description,will be better understood when read in conjunction with the appendeddrawings. For the purpose of illustration only, there is shown in thedrawings certain embodiments. It's understood, however, that theinventive concepts disclosed herein are not limited to the precisearrangements and instrumentalities shown in the figures.

FIGS. 1A-1B illustrate a schematic and cross-sectional microstructure ofa prior art thin film high-temperature superconducting tape.

FIG. 2 illustrates an apparatus for mechanically abrading a tapesubstrate, in accordance with an embodiment.

FIG. 3 illustrates critical current measurements of tapes without copperstabilizers, before and after substrate abrasion, in accordance with anembodiment.

FIG. 4 illustrates an ultra-thin film HTS tape wound around a brasscore, in accordance with an embodiment.

FIG. 5 illustrates critical current measurements for an ultra-thin filmHTS tape wound around a brass core, in accordance with an embodiment.

FIG. 6 illustrates an ultra-thin film HTS tape wound around a brasscore, in accordance with an embodiment.

FIG. 7 illustrates critical current measurements for standard HTS tapeshaving copper stabilizer, in accordance with an embodiment.

FIG. 8 illustrates critical current measurements for an ultra-thin filmHTS tape wound around a brass core, in accordance with an embodiment.

FIG. 9 illustrates critical current measurements for an ultra-thin filmHTS tape wound around a brass core from 3.17 mm diameter down to 0.51 mmdiameter, in accordance with an embodiment.

FIG. 10A illustrates a rare earth barium copper oxide (REBCO) wire madeby spiral winding six layers of 45 μm thick tapes over a 0.8 mm diameterformer.

FIG. 10B illustrates the REBCO wire shown in FIG. 10A after being bentto a diameter of 3 cm.

FIG. 11 is a flowchart illustrating an embodiment of a method forfabricating a superconducting tape, in accordance with an embodiment.

FIG. 12 illustrates a schematic and cross-sectional microstructure of anultra-thin film high-temperature superconducting tape without theimplementation of a copper layer, in accordance with an embodiment.

FIG. 13 illustrates a schematic and cross-sectional microstructure of anultra-thin film high-temperature superconducting tape including a copperlayer on the silver overlayer, in accordance with an embodiment.

BRIEF SUMMARY

Embodiments are directed to a superconductor tape. The superconductortape comprises: a substrate; a buffer layer overlying the substrate; asuperconductor layer overlying the buffer layer; and an overlayeroverlying the superconductor layer. The superconductor tape has athickness in a range of 15-30 μm.

In an embodiment, the substrate has a thickness in a range of 10 to 28μm, the buffer layer has a thickness in a range of 0.1 to 2 μm, thesuperconductor layer has a thickness in a range of 0.5 to 5 μm, and theoverlayer has a thickness in a range of 0.2 to 4 μm.

In an embodiment, the overlayer comprises silver (Ag) or gold (Au).

In an embodiment, the superconductor layer comprises a material selectedfrom the group consisting of REBa₂Cu₃O_(7-x), where RE is one of moreelements selected from the group consisting of Y, Gd, Sm, Nd, Eu, Dy,Ho, Yb, Er, Tm, and Lu, and wherein 0≤x≤1.

In an embodiment, the buffer layer comprises a material selected fromthe group consisting of MgO, LaMnO₃, CeO₂, Gd₂Zr₂O₇, YSZ, SrTiO₃, andcombinations thereof.

In an embodiment, the substrate comprises a metal selected from thegroup consisting of Hastelloy, Stainless Steel, Ni-W, Inconel, metallicglasses, and combinations thereof.

In an embodiment, the superconducting tape has an engineering currentdensity of at least 700 A/mm² at 77 K.

In an embodiment, the superconductor tape may further comprise aconductive layer overlying the overlayer. The conductive layer may havea thickness of 2 μm to 50 μm yielding an overall tape thickness of 15 to80 μm.

In an embodiment, the conductive layer comprises copper (Cu) or at leastone Cu alloy or a conductive metal alloy.

In an embodiment, the superconductor tape may further comprise a secondoverlayer below the substrate, and a second conductive layer below thesecond overlayer.

Embodiments are also directed to a superconductor wire. Thesuperconductor wire comprises: a core having a diameter of at least 0.5mm; and a stack of superconductor tapes spiral wound around the core.The stack of superconductor tapes have an outside diameter between 0.7mm and 3 mm while wound around the core.

Embodiments are further directed to a method for fabricating asuperconducting tape. The method comprises: providing a substrate;depositing a buffer layer on the substrate; forming a superconductinglayer on the buffer layer; placing an overlayer on the superconductinglayer; and removing a portion of the substrate subsequent thedepositing, forming, placing, and positioning steps, whereby a thicknessof the superconducting tape is reduced to 15-80 μm.

In an embodiment, the removing is performed via a mechanical abradingprocess such as by using at least one grinding wheel, via a chemicaletching process, or via a electrolytic etching process.

In an embodiment, subsequent to the removing step, the substrate has athickness in a range of 10-75 μm, the deposited buffer layer has athickness in a range of 0.1-2 μm, the formed superconductor layer has athickness in a range of 0.5-5 μm, and the placed overlayer has athickness in a range of 0.5-4 μm.

In an embodiment, the overlayer comprises silver (Ag) or gold (Au).

In an embodiment, the superconductor layer comprises a material selectedfrom the group consisting of REBa₂Cu₃O_(7-x), where RE is one of moreelements selected from the group consisting of Y, Gd, Sm, Nd, Eu, Dy,Ho, Yb, Er, Tm, and Lu, and wherein 0≤x≤1.

In an embodiment, the buffer layer comprises a material selected fromthe group consisting of MgO, LaMnO₃, CeO₂, Gd₂Zr₂O₇, YSZ, SrTiO₃, andcombinations thereof.

In an embodiment, the substrate comprises a metal selected from thegroup consisting of Hastelloy, Stainless Steel, Ni-W, Inconel, metallicglasses, and combinations thereof.

In an embodiment, the superconducting tape has an engineering currentdensity of at least 250 A/mm² at 77 K, subsequent the removing step.

In an embodiment, the method may further comprise positioning aconductive layer on the overlayer, wherein the conductive layer has athickness of 2 μm to 50 μm yielding an overall tape thickness of 15 to80 μm.

In an embodiment, the conductive layer comprises copper (Cu) or at leastone Cu alloy or a conductive metal alloy.

In an embodiment, the method may further comprise placing a secondoverlayer below the substrate, and positioning a second conductive layerbelow the second overlayer.

DETAILED DESCRIPTION

It is to be understood that the figures and descriptions of the presentinvention may have been simplified to illustrate elements that arerelevant for a clear understanding of the present invention, whileeliminating, for purposes of clarity, other elements found in a typicalsuperconductor tape or typical method for fabricating a superconductortape. Those of ordinary skill in the art will recognize that otherelements may be desirable and/or required in order to implement thepresent invention. However, because such elements are well known in theart, and because they do not facilitate a better understanding of thepresent invention, a discussion of such elements is not provided herein.It is also to be understood that the drawings included herewith onlyprovide diagrammatic representations of the presently preferredstructures of the present invention and that structures falling withinthe scope of the present invention may include structures different thanthose shown in the drawings. Reference will be made to the drawingswherein like structures are provided with like reference designations.

Before explaining at least one embodiment in detail, it should beunderstood that the inventive concepts set forth herein are not limitedin their application to the construction details or componentarrangements set forth in the following description or illustrated inthe drawings. It should also be understood that the phraseology andterminology employed herein are merely for descriptive purposes andshould not be considered limiting.

It should further be understood that any one of the described featuresmay be used separately or in combination with other features. Otherinvented devices, systems, methods, features, and advantages will be orbecome apparent to one with skill in the art upon examining the drawingsand the detailed description herein. It's intended that all suchadditional devices, systems, methods, features, and advantages beprotected by the accompanying claims.

For purposes of this disclosure, the terms “film” and “layer” may beused interchangeably.

It is an objective of the embodiments described herein to achieve anultra-thin film HTS tape.

In an embodiment, an ultra-thin film HTS tape includes an overallthickness without copper stabilizer in the range of 15-30 μm, andpreferably −25 μm. In another embodiment, an ultra-thin film HTS tapemay be achieved by reducing the tape's substrate thickness after thesuperconductor film is deposited on the substrate. In still anotherembodiment, an ultra-thin film HTS tape may exhibit an engineeringcurrent density in the range of 700-16,000 A/mm², and preferably above800 A/mm² at 77K, in a zero applied field. In yet still anotherembodiment, a critical current of 210 A/cm-width with a 30 micron tapeyields a current density (J_(e)) of ˜700 A/mm².

In one embodiment, an ultra-thin film HTS tape may be achieved bymechanically abrading or chemically etching the substrate of an HTS tapeafter the silver overlayer and superconductor films are deposited on thesubstrate. FIG. 2, by way of example only, illustrates an apparatus formechanically abrading an HTS's substrate. The substrate side of the tapemay be passed over one or more abrasive grinding wheels that includeappropriate grit sizes. For example, in one embodiment the wheels may bemade of one or more of silicon carbide and diamond having grit sizes of48, 60, 80, and 120. The grinding wheels may remove portions of thesubstrate via mechanical abrasion. Each wheel may have the same grit orsuccessively different grit sizes.

Copper layer may preferably be added after the substrate removal step.Alternatively, the tape may be initially provided with a copperstabilizer on the superconductor side only (i.e., on the silveroverlayer) or even on both sides (i.e., the superconductor side andsubstrate side). This latter approach would then require removing boththe substrate portion and copper that was previously formed on thesubstrate.

In yet another embodiment, the grinding process is conducted in both theforward and reverse directions in order to uniformly abrade thesubstrate along the HTS tape length and width. In still anotherembodiment, the speed of the grinding wheels, the tape movement speed,and/or the tape tension are optimized in combination to maintain theperformance and uniformity of the HTS tape. For example, in oneembodiment, the wheels may be rotated between 100 and 2000 rpm. Inanother embodiment, the linear speed of the tape may range from 5 m/h to200 m/h. In yet another embodiment, the tape tension may range from 2 Nto 100 N.

In an embodiment, abrasion of the substrate of an HTS tape, inaccordance with any of the aforementioned embodiments, may remove ˜20 to40 μm substrate from a standard HTS tape having an initial overallthickness of 55 μm including a silver overlayer and with no copperstabilizer, resulting in an HTS tape with a thickness of ˜15 to 35 μm.In yet another embodiment, abrading the substrate of an HTS tape with aninitial critical current of 250 A/cm-width at 77 K, 0 T, in accordancewith any of the aforementioned embodiments, may result in HTS tapes withengineering current densities between about 700 A/mm² and 1650 A/mm².Abrading the substrate of an HTS tape with an initial critical currentof 500 A/cm-width at 77 K, 0 T, in accordance with any of theaforementioned embodiments, may result in HTS tapes with engineeringcurrent densities between about 1400 A/mm² and 3300 A/mm². Similarly,abrasion of the substrate of an HTS tape, in accordance with any of theaforementioned embodiments, may remove ˜25 to 75 μm substrate from a 105μm thick HTS tape with a 100 μm substrate including a silver overlayerand without a conductive layer, resulting in an HTS tape with athickness ˜30 to 80 μm. Thus, depending on the removal technique,thickness of the superconducting tape may be reduced to 15-80 μm.

FIG. 3, by way of example only, illustrates critical currentmeasurements of HTS tapes including a silver overlayer and withoutcopper stabilizers, before and after substrate abrasion in accordancewith the above-described embodiments. The tapes are tested at 77K in azero applied magnetic field. In an embodiment, a substantial reductionin HTS tape thickness may be achieved without impacting the criticalcurrent density of the super conducting tape. For example, the criticalcurrent of the tape before and after substrate abrasion is virtuallyidentical at 378 A/12 mm width, which corresponds to an engineeringcurrent density of 1260 A/mm². This is significantly higher than theengineering critical current density of the original tape, or 572 A/mm².

Ultra-thin HTS tapes made in accordance with any of the aforementionedprocesses may be used to fabricate high current density wires or cablesby winding the tape around cores (such as brass or other metal) havingsmall diameters in the range of approximately 0.5 mm to 2 mm. In oneembodiment, after the substrate removal process, 5 μm of copperstabilizer may be electrodeposited onto the silver overlayer of a 2 mmwide, 25 μm thick HTS tape made in accordance with any of theaforementioned processes (i.e., resulting in a 30 μm thick ultra-thinfilm HTS tape).

FIG. 4, by way of example only, illustrates a 30 μm thick ultra-thinfilm HTS tape (including a silver overlayer and overlying copper layer)wound around a brass core having a ˜2.16 mm diameter. In an embodiment,the critical current of an ultra-thin film HTS tape is substantially thesame when the tape is flat or when the tape is wound around a brasscore. FIG. 5, by way of example only, illustrates critical currentmeasurements for the ultra-thin film HTS tape described in FIG. 4 laidflat, wound around a 3.07 mm diameter brass core, and wound around a2.16 mm diameter brass core. The tapes are tested at 77 K in azero-applied magnetic field. In embodiments, the three differentarrangements of the tape exhibit substantially the same criticalcurrent. For these three arrangements, the critical current remainedunchanged between 47-48 A in the 2 mm wide tapes.

Many typical superconductor tapes include ˜20 μm thick of copperstabilizer on the superconducting side of the tape. In anotherembodiment, 20 μm of copper stabilizer may be electrodeposited onto a 2mm wide, 25 μm thick HTS tape made in accordance with theafore-mentioned processes (i.e., resulting in a 45 μm thick ultra-thinfilm HTS tape). In this case, copper may be deposited on thesuperconductor side atop the silver overlayer, and a lesseramount/thickness of copper may optionally also be deposited on thesubstrate side. This ultra-thin film HTS tape may be wound around brasscores having small diameters in the range of approximately 0.5 mm to 2mm. FIG. 6, by way of example only, illustrates a 45 μm thick ultra-thinfilm HTS tape wound around a brass core with a ˜1.1 mm diameter.

FIG. 7, by way of example only, illustrates critical currentmeasurements for standard 2 mm wide HTS tapes having ˜20 μm thick ofcopper stabilizer and a total thickness of ˜75 μm. The tapes are laidflat and wound around brass cores having diameters between approximately2.06 mm and 12.5 mm. The tapes are tested at 77 K in a zero appliedmagnetic field. In an embodiment, the critical current remains unchangedbetween 6.5 mm and 12.5 mm. However, the critical current drops by 42%when the tape is wound around a brass core having a diameter of 3.17 mm,and drops by a total of 85% when the tape is wound around a brass corehaving a diameter of 2.06 mm.

FIG. 8, by way of example only, illustrates critical currentmeasurements for the ultrathin film HTS tape described in FIG. 6 andmade in accordance with any of the aforementioned processes. The tapesare laid flat, or wound around brass cores having diameters betweenapproximately 1.1 mm and 12.5 mm. The tapes are tested at 77 K in a zeroapplied magnetic field. Unlike the standard tapes in FIG. 7, ultra-thinfilm HTS described herein maintain substantially the same criticalcurrent all the way down to the 1.1 mm diameter bend.

In an embodiment, substantially small diameter HTS wires in the range ofapproximately 0.7 to 3 mm in outer diameter may be manufactured usingthe ultra-thin film HTS tapes described herein. In another embodiment,HTS wires having diameters between approximately 0.7 mm to 5 mm may bemanufactured using the ultra-thin film HTS tapes described herein.

Ultrathin superconductor tapes have been tested at even smaller benddiameters. FIG. 9, by way of example only, shows current-voltagecharacteristics of I_(c) measurements of 2 mm wide, 25 μm thick REBCOtapes (including a silver overlayer) that underwent substrate removalvia any of the aforementioned processes, and with an additional 20 μmlayer of Cu stabilizer on the superconductor side atop the silveroverlayer (resulting in a total superconducting tape thickness of 45 μm)when bent around brass cores from 3.17 mm diameter down to 0.51 mmdiameter. Remarkably, the ultrathin superconductor tapes exhibited nodegradation in critical current even when wound on 0.51 mm diametercores. Another copper layer may optionally also be provided in a lesseramount/thickness on the substrate side. To facilitate copper depositionon the substrate side, a thin layer (˜0.5 μm thick) Ag layer mayoptionally be deposited first.

The ultrathin tapes may be bent on very small formers/cores discussedabove may now be utilized to fabricate round, small-diameter REBCOwires. Round REBCO wires have been successfully fabricated with sixlayers of 2 mm wide, 45 μm thick tapes (including a 3 μm thick silveroverlayer and an overlying 20 μm thick Cu stabilizer only on thesuperconductor side) on a 0.8 mm diameter former/core as shown in FIG.10A which, by way of example only, illustrates a 1.6 mm outside diameterround REBCO wire made by spiral winding six layers of 45 μm thick tapesover a 0.8 mm diameter former. A pitch of 2.8 mm was used. This roundREBCO wire was able to sustain a critical current of 283 A at 77 K, 0 Twhich corresponds to a current density (J_(e)) of 140 A/mm².Additionally, no degradation in critical current (I_(e)) was found evenwhen the wire was bent around a diameter of 3 cm. FIG. 10B, by way ofexample only, illustrates the round wire after being bent to a diameterof 3 cm.

FIG. 11 is a flowchart illustrating an embodiment of a method 1100 forfabricating a superconductor tape. In an embodiment, a substrate may beprovided (block 1102). Examples of substrates may include Hastelloy,Stainless Steel, Ni-W, Inconel, and metallic glasses. A buffer layer maythen be deposited on the substrate (block 1104). Examples of bufferlayers may include MgO, LaMnO₃, CeO₂, Gd₂Zr₂O₇, YSZ and SrTiO₃. Asuperconducting layer is then formed on the buffer layer (block 1106)via a process such as Metal Organic Chemical Vapor Deposition, PulsedLaser Deposition, Reactive Co-evaporation, Metal Organic Deposition andChemical Solution Deposition. Examples of superconducting layers mayinclude REBa₂Cu₃O_(7-x) (REBCO, RE=rare earth including Y, Gd, Sm, Nd,Eu, Dy, Ho, Yb, Er, Tm, Lu). Next, an overlayer may be placed on thesuperconducting layer (block 1108). Examples of overlayers may includesilver (Ag) or Gold (Au). After the depositing, forming, and placing,steps (i.e., blocks 1104, 1106, 1108) are performed, a portion of thesubstrate is then removed (block 1110), whereby a thickness of thesuperconducting tape may be reduced to 15-80 μm. The removal of theportion of the substrate is performed via a process selected from thegroup consisting of a mechanical abrading process, a chemical etchingprocess, an electrolytic etching process, and combinations thereof.Next, an optional conductive layer may be positioned on the overlayer orall around the tape. Examples of conductive layers may include copper(Cu), or at least one copper alloy or a conductive metal alloy. Tofacilitate copper deposition on the substrate side, a thin layer (˜0.5μm thick) Ag layer may optionally be deposited first. Alternatively,after removal of a portion of the substrate, the tape may be slit tonarrow its width, deposited with a second thin layer of silver on thesuperconductor side and edges followed by conductive layer depositionatop the second silver layer. It may be preferable to deposit the copperlayer on the superconductor side atop the second silver overlayer.Another conductive layer may optionally also be provided in a lesseramount/thickness on the substrate side. To facilitate conductive layerdeposition on the substrate side, a thin layer (˜0.5 μm thick) Ag layermay optionally be deposited first. It is noted that there preferably mayalways be an overlayer layer such as silver on the superconductor sideprior to substrate portion removal.

Although the embodiments in method 1100 are described above withreference to deposition, forming, and placing of various layers, thedeposition, forming, and placing of any or all of the above-mentionedlayers is hereby defined to include epitaxial growth (or any other layerformation technique described above or otherwise known to one skilled inthe art) of any of the layers. Furthermore, it's understood that method1100 may implement any of the materials and processes described abovewith respect to FIG. 1.

While results on thin superconductor tapes prepared by a mechanicalabrasion method have been disclosed, similar results are expected inthin profile superconductor tapes prepared by chemical etching of thesubstrate after deposition of the superconductor layer and overlayer.For example, in one embodiment, etching of the Hastelloy substrate couldbe performed using a solution of, for example, hydrochloric acid, aceticacid and nitric acid. After etching, the HTS tape may be cleaned with,for example, water and methanol and then dried. Immersion etching ofHastelloy substrate is feasible using a solution of, for example,chromic acid and hydrochloric acid. In another embodiment, electrolyticetching of the Hastelloy substrate may also be performed using asolution of, for example, oxalic acid and hydrochloric acid. In stillanother embodiment, electrolytic etching may also be done in a cellcontaining, for example, a carbon cathode and a stainless steel anode.

There may be at least two optional configurations for using theultra-thin superconductor tape. In a first option, the ultra-thin tapemay be used as is after substrate removal or with a conductivestabilizer layer. The conductive stabilizer layer may be deposited onthe silver overlayer atop the superconductor layer. The conductivestabilizer layer may also be deposited on the substrate side aftersubstrate removal. In this case, this would most likely require adeposition of a thin silver layer (<0.5 μm) on the substrate side forease of conductive layer deposition on the substrate side. This optionmay be used for employing the ultra-thin tapes as flat tapes. In asecond option, after substrate removal, a slit of the tape to narrow thetape width (˜2 mm) is performed if necessary, followed by a deposit of asecond thin silver layer (<0.5 μm) on the superconducting side, and thena deposit of a conductive layer stabilizer on this silver layer on thesuperconducting side. Also, after slitting following substrate removal,silver needs to be deposited to protect the edges. By slitting theultra-thin tapes, this option may be used for converting the ultra-thintapes into round wires. The starting superconductor tape has a silverlayer on the superconducting side prior to substrate removal. In bothoptions, a thin silver layer would likely be deposited after substrateremoval to facilitate copper electrodeposition if Cu is deposited on allsides.

FIG. 12 illustrates a schematic architecture of an ultra-thin filmhigh-temperature superconducting tape without the implementation of aconductive layer, in accordance with an embodiment.

FIG. 13 illustrates a schematic architecture of an ultra-thin filmhigh-temperature superconducting tape including a conductive layer (inthis case a copper layer) on the silver overlayer, in accordance with anembodiment.

While FIG. 12 illustrates a superconducting tape without a conductivelayer and FIG. 13 illustrates a superconducting tape with a copperlayer, there may be many different alternative configurations. Somealternative examples are as follows: no Cu after substrate removal, Cudeposition on superconductor side only after substrate removal, Cudeposition all around the tape after substrate removal plus startingwith a thicker substrate (for example, ˜100 μm instead of ˜50 μm) alongwith the different Cu options. FIG. 12 and FIG. 13 respectivelyillustrate no Cu after substrate removal and Cu on superconductor sideonly after substrate removal. A preferred embodiment at this time is a45 μm thick tape with a 20 μm Cu layer on the superconductor side aftersubstrate removal to reduce tape thickness from 55 μm to 25 μm.

Embodiments are directed to a superconductor tape. The superconductortape comprises: a substrate; a buffer layer overlying the substrate; asuperconductor layer overlying the buffer layer; and an overlayeroverlying the superconductor layer. The superconductor tape has athickness in a range of 15-30 μm.

In an embodiment, the substrate has a thickness in a range of 10 to 28μm, the buffer layer has a thickness in a range of 0.1 to 2 μm, thesuperconductor layer has a thickness in a range of 0.5 to 5 μm, and theoverlayer has a thickness in a range of 0.2 to 4 μm.

In an embodiment, the overlayer comprises silver (Ag) or gold (Au).

In an embodiment, the superconductor layer comprises a material selectedfrom the group consisting of REBa₂Cu₃O_(7-x), where RE is one of moreelements selected from the group consisting of Y, Gd, Sm, Nd, Eu, Dy,Ho, Yb, Er, Tm, and Lu, and wherein 0<x<1.

In an embodiment, the buffer layer comprises a material selected fromthe group consisting of MgO, LaMnO₃, CeO₂, Gd₂Zr₂O₇, YSZ, SrTiO₃, andcombinations thereof.

In an embodiment, the substrate comprises a metal selected from thegroup consisting of Hastelloy, Stainless Steel, Ni-W, Inconel, metallicglasses, and combinations thereof.

In an embodiment, the superconducting tape has an engineering currentdensity of at least 700 A/mm² at 77 K.

In an embodiment, the superconductor tape may further comprise aconductive layer overlying the overlayer. The conductive layer may havea thickness of 2 μm to 50 μm yielding an overall tape thickness of 15 to80 μm.

In an embodiment, the conductive layer comprises copper (Cu) or at leastone Cu alloy or a conductive metal alloy.

In an embodiment, the superconductor tape may further comprise a secondoverlayer below the substrate, and a second conductive layer below thesecond overlayer.

Embodiments are also directed to a superconductor wire. Thesuperconductor wire comprises: a core having a diameter of at least 0.5mm; and a stack of superconductor tapes spiral wound around the core.The stack of superconductor tapes have an outside diameter between 0.7mm and 3 mm while wound around the core.

Embodiments are further directed to a method for fabricating asuperconducting tape. The method comprises: providing a substrate;depositing a buffer layer on the substrate; forming a superconductinglayer on the buffer layer; placing an overlayer on the superconductinglayer; and removing a portion of the substrate subsequent thedepositing, forming, placing, and positioning steps, whereby a thicknessof the superconducting tape is reduced to 15-80 μm.

In an embodiment, the removing is performed via a mechanical abradingprocess such as by using at least one grinding wheel, via a chemicaletching process, or via a electrolytic etching process.

In an embodiment, subsequent to the removing step, the substrate has athickness in a range of 10-75 μm, the deposited buffer layer has athickness in a range of 0.1-2 μm, the formed superconductor layer has athickness in a range of 0.5-5 μm, and the placed overlayer has athickness in a range of 0.5-4 μm.

In an embodiment, the overlayer comprises silver (Ag) or gold (Au).

In an embodiment, the superconductor layer comprises a material selectedfrom the group consisting of REBa₂Cu₃O_(7-x), where RE is one of moreelements selected from the group consisting of Y, Gd, Sm, Nd, Eu, Dy,Ho, Yb, Er, Tm, and Lu, and wherein 0<x<1.

In an embodiment, the buffer layer comprises a material selected fromthe group consisting of MgO, LaMnO₃, CeO₂, Gd₂Zr₂O₇, YSZ, SrTiO₃, andcombinations thereof.

In an embodiment, the substrate comprises a metal selected from thegroup consisting of Hastelloy, Stainless Steel, Ni-W, Inconel, metallicglasses, and combinations thereof.

In an embodiment, the superconducting tape has an engineering currentdensity of at least 250 A/mm² at 77 K, subsequent the removing step.

In an embodiment, the method may further comprise positioning aconductive layer on the overlayer, wherein the conductive layer has athickness of 2 μm to 50 μm yielding an overall tape thickness of 15 to80 μm.

In an embodiment, the conductive layer comprises copper (Cu) or at leastone Cu alloy or a conductive metal alloy.

In an embodiment, the method may further comprise placing a secondoverlayer below the substrate, and positioning a second conductive layerbelow the second overlayer.

Although embodiments are described above with reference to an overlayercomprising silver (which may be optional), overlayers comprising anysuitable composition such as silver or gold may alternatively beemployed as part of the superconducting tape, and may therefore utilizethe advantages of the configurations and embodiments described above.

Also, although embodiments are described above with reference to aconductive layer comprising copper (which may be optional), conductivelayers comprising any suitable composition such as copper or copperalloys or conductive metal alloys may alternatively be employed as partof the superconducting tape, and may therefore utilize the advantages ofthe configurations and embodiments described above.

Features in any of the embodiments described above may be employed incombination with features in other embodiments described above, suchcombinations are considered to be within the spirit and scope of thepresent invention.

The contemplated modifications and variations specifically mentionedabove are considered to be within the spirit and scope of the presentinvention.

It's understood that the above description is intended to beillustrative, and not restrictive. The material has been presented toenable any person skilled in the art to make and use the conceptsdescribed herein, and is provided in the context of particularembodiments, variations of which will be readily apparent to thoseskilled in the art (e.g., some of the disclosed embodiments may be usedin combination with each other). Many other embodiments will be apparentto those of skill in the art upon reviewing the above description. Thescope of the embodiments herein therefore should be determined withreference to the appended claims, along with the full scope ofequivalents to which such claims are entitled. In the appended claims,the terms “including” and “in which” are used as the plain-Englishequivalents of the respective terms “comprising” and “wherein.”

EXAMPLES

Example 1. Proof of concept experiments were conducted in alaboratory-scale reel-to-reel apparatus with two grinding wheels thatrotated at a speed of 240 RPM in opposite directions (clockwise andcounter-clockwise). The rear substrate surface of a 0.055 mm thick HTStape including a silver overlayer was abraded over the two 60 gritgrinding wheels, at a linear tape speed of about 10 cm/min. After eachpass of mechanical abrasion, the direction of the tape was reversed.After 12 passes, the mechanical abrasion was continued with two 120 gritgrinding wheels at a linear tape speed of about 10 cm/min. The tapedirection was reversed after each pass and a total of four passes werecompleted. The thickness of the tape was measured at different locationsalong the tape width and length. The thickness of the HTS tape was foundto be reduced to 0.02-0.025 mm. The critical current of the tape wastested using transport current in a four-probe set up. The criticalcurrent of the ground ˜0.025 mm tape was measured to be the same as thecritical current of the parent 0.055 mm tape, at 360 A.

Example 2. The experiment detailed above in Example 1 was repeated withanother 0.055 mm thick HTS tape including a silver overlayer that wasmechanical abraded on the rear substrate side at a linear tape speed ofabout 10 cm/min. In this example, the tape was abraded in 10 passes(with reversal of direction after each pass) over two 60 grit grindingwheels. Then the 60 grit wheels were replaced with two 80 grit grindingwheels and the tape was further abraded in 9 passes (with reversal ofdirection after each pass). The thickness of the HTS tape was found tobe reduced to 0.02-0.025 mm. The critical current of the ground ˜0.025mm tape was measured to be the same as the critical current of theparent 0.055 mm tape, at 378 A.

Example 3. A HTS tape including a silver overlayer (with no copper) onthe superconductor side was mechanically-abraded to 0.025 mm thicknessas described in either of the two examples above. The HTS tape was thenslit to 2 mm width. Another (second) thin layer of silver (<1 μm) wasdeposited on the slit tape atop the original silver layer on thesuperconductor side. The second silver layer is deposited to cover,inter alia, the slit edges as well as to facilitate copper deposition onthe superconductor side. A 20 μm thickness of copper was electroplatedon the silver surface of the tape atop the second/another silver layeron the superconductor side using a copper nitrate solution. The solutioncontained 100 g of copper nitrate in 1 L of methanol. A plating currentof 400 mA and a plating time of 5 minutes were used. The thickness ofthe copper-plated tape was measured to be 0.045 mm. The critical currentof the tape was measured using transport current with a four-probemethod. The critical current at 77 K, zero field was measured to be 59A.The copper-plated tape was wound around a 1.1 mm diameter brass core andthe critical current was measured again in a bent state. The criticalcurrent of the wound tape was found to be almost 59 A indicating nodegradation was present. It is noted that another silver and copperlayer may optionally be added to the substrate side after substrateportion removal as described in the embodiments above.

What is claimed is:
 1. A superconductor wire comprising: a core having adiameter of at least 0.5 mm; and a stack of superconductor tapes spiralwound around the core; wherein the stack of superconductor tapes have anoutside diameter between 0.7 mm and 3 mm while wound around the core. 2.The superconductor wire of claim 1, wherein each superconductor tapewithin the stack of superconductor tapes comprises a superconductorlayer comprising REBCO.
 3. The superconductor wire of claim 1, whereineach superconductor tape within the stack of superconductor tapescomprises a superconductor layer and an overlayer overlying thesuperconductor layer.
 4. The superconductor wire of claim 3, whereineach overlayer comprises silver (Ag) or gold (Au).
 5. The superconductorwire of claim 3, further comprising a stabilizer layer overlying eachoverlayer.
 6. The superconductor wire of claim 5, wherein eachstabilizer layer comprises copper (Cu).
 7. The superconductor wire ofclaim 1, wherein the core comprises brass.
 8. The superconductor wire ofclaim 1, wherein the superconductor wire exhibits no degradation incritical current as compared to the superconductor wire prior to thestack of superconductor tapes being spiral wound around the core.